There is currently widespread interest in coherent technologies due to the increasing requirements for transmission-capacity in wavelength division multiplexing (WDM) systems. For instance, coherent optical fiber communication systems are known to allow for the utilization of high spectral efficiency modulation formats such as multilevel quadrature amplitude modulation or m-ary phase shift keying.
Quadrature (IQ) modulators generally output modulated transmission signals on two phase-shifted carrier waves. The two carrier waves are out of phase with each other by 90 and are thus called quadrature carriers or quadrature components. The modulated waves are summed, and the final waveform is a combination of both phase-shift keying (PSK) and amplitude-shift keying (ASK). Coherent Quadrature PSK (QPSK) and Quadrature Amplitude Modulators (QAM) are routinely implemented in optical communication systems using a pair of Mach-Zehnder modulators (MZM), the output of one being phase-shifted 90 degrees relative to the output of the other to provide each of the required quadrature components of the combined IQ modulator. Common implementations include known travelling wave Mach-Zehnder (TWMZ) modulators
For instance, Mach-Zehnder type modulators are commonly nested in a variety of different material platforms (e.g., InP, GaAs, Si, and LiNbO3) to generate the coherent optical transmission signals in coherent quadrature modulation systems. The existing Mach-Zehnder type modulators are usually long (e.g., from 5 mm up to a few centimeters) because of the relatively weak electro-optic effect involved, and thus require sophisticated traveling-wave designs for high frequency operation.
Nested electro-absorption modulators have also been demonstrated to generate an optical differential quadrature phase shift keying (DQPSK) transmission signal, such as described in U.S. Pat. No. 7,403,670.
While some have proposed the use of a single ring resonator modulator to replace current Mach-Zehnder modulators to achieve PSK modulation, the proposed implementations suffer several drawbacks, such as relatively high operation losses, nonlinear phase transitions unsuitable for coherent transmission implementations, and a general unsuitability for high-order modulation formats such as 4-PSK and 16QAM. Even for relatively simple modulation formats like QPSK, in the context of long haul optical transmission, a linear response of the optical transmitter is vital in order to enable advanced signal processing such as pre-emphasizing and pulse shaping, among others.
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